A layered composite material comprised of layers of an alloy and a process for producing the layered composite material.
Gold-tin (Auxe2x80x94Sn) eutectic solders are commonly used in the optoelectronic and microelectronic industries for chip bonding to dies. Auxe2x80x94Sn solder is classified as a xe2x80x9chard solderxe2x80x9d with superior mechanical and thermal properties relative to xe2x80x9csoftxe2x80x9d solders, such as the Pbxe2x80x94Sn system.
Auxe2x80x94Sn solder can be applied in a number of ways, i.e., as Auxe2x80x94Sn preforms, solder paste, by sequential evaporation and sequential electrodeposition. Compared with solder preforms and pastes, evaporated solder is cleaner and provides more precise thickness and positional control. Thin film technology, however, involves expensive vacuum systems.
Electroplating of Auxe2x80x94Sn eutectic solder is an attractive alternative in that it is a low cost process, offering the thickness and positional control of thin film techniques. Auxe2x80x94Sn solder layers have been produced sequentially by depositing Au first on a seed layer, followed by Sn (see for example C. Kallmayer, D. Lin, J Kloeser, H. Oppermann, E. Zakel and H. Reichl, 1995 IEEE/CPMT International Electronics Manufacturing Technology Symposium, (1995) 20; C. Kallmayer, D. Lin, H. Oppermann, J. Kloeser, S. Werb, E. Zakel and H. Reichl, 10th European Microelectronics Conference, (1995) 440; and E. Zakel and H. Reichl, Chapter 15, in Flip-Chip Technologies, ed., J. Lau, McGraw-Hill, (1995) 415.
Commercially available Au and Sn baths are utilized from which several microns of solder can be deposited sequentially. Co-electrodeposition or codeposition of Au and Sn from a single solution offers the same economic advantage of sequential plating relative to vacuum deposition techniques, as well as the prospect of depositing the solder in a single step without oxidation of an outer Sn layer.
One of the challenges with Auxe2x80x94Sn alloy plating baths is preventing the oxidation of Sn(II) to Sn(IV), as discussed in D. R. Mason, A. Blair and P. Wilkinson, Trans. Inst. Met. Finish., 52 (1974) 143. Oxidation of Sn can be minimized by using soluble Sn anodes. However, Au is deposited on the anodes unless they are isolated by semi-permeable diaphragms.
It has been reported that Auxe2x80x94Sn alloys containing up to 30 at (i.e. atomic) % Sn could be deposited from baths containing no free cyanide, and containing the Sn as its stannate complex formed with KOH (see E. Rau and K. Bihlimaier, Galvanische Weissgolniederschlage, Mitt. Forschungsinst. Probierants. Edelmetalle Staatl. Hoheren Fachschule Schwab. Gmund, 11 (1937) 59. Later claims concerning Auxe2x80x94Sn alloy plating, however, have been based on the use of alkaline and acid cyanide electrolytes, where Sn in many cases has been incorporated with the goal of obtaining brightening effects rather than producing deposits with significant amounts of Sn.
Several cyanide based systems have been reported (see T. Frey and W. Hempel, DE 4406434, (1995); W. Kuhn, W. Zilske and A.-G. Degussa, Ger. DE 4,406,434, Aug. 10, 1995: N Kubota, T. Horikoshi and E. Sato, J. Met. Fin. Soc. Japan, 34 (1983) 37; and Y. Tanabe, N. Hasegawa and M. Odaka, J. Met. Fin. Soc. Japan, 34 (1983) 8.
Frey and Hempel developed a bright Auxe2x80x94Sn plating bath with a pH of 3-14, comprised of potatassium dicyanoaurate, soluble Sn(IV), potassium hydroxide, potassium salt of gluconic, glucaric and/or glucaronic acid, conductivity salt, piperazine and a small amount of As. The bath was used to plate small parts with an alloy containing 5-25 wt % Sn. Bright deposits were obtained for thicknesses greater than 0.1 xcexcm and the solution exhibited long term stability without the use of soluble Sn anodes.
A.-G. Degussa, Ger. DE 4,406,434 teaches using potassium dicyanoaurate and tin chloride and claims a deposit composition of 8 wt % Sn and thickness of 5 xcexcm.
Auxe2x80x94Sn codeposition from a cyanide system using pyrophosphate as a buffering agent was studied by Kubota et al (N. Kubota, T. Horikoshi and E. Sato, J. Met. Fin. Soc. Japan, 34 (1983) 37; and N. Kubota, T. Horikoshi and E. Sato, Plating and Surface Finishing, 71 (1984) 46. The basic formula consisted of K4P2O7, Kau(CN)2 and SnCl2xe2x80x942H2O. The mass transfer was investigated to clarify reaction mechanisms between monovalent Au or bivalent Sn and pyrophosphate ions, by measuring conductivity, kinematic viscosity and limiting current density of the bath components. Two pyrophosphate ions were complexed with one stannous ion, with excess pyrophosphate acting as a supporting constituent.
Tanabe et al, referred to above, obtained various Auxe2x80x94Sn alloy compositions by electrodeposition from cyanide baths containing HauCl4xe2x80x944H2O, K2SnO3xe2x80x943H2O, KCN and KOH. Although a linear relationship was not found between the Sn content in the bath and the Sn content in the alloy formed, a relationship was found between the two alloys which permitted formation of alloys of desired compositions. The composition of electrodeposited Auxe2x80x94Sn was shifted by about 10% to the Sn side in comparison with alloys at thermal equilibrium; thus exhibiting the xcex6 phase in the 25-29 at % range. AuSn, AuSn2 and AuSn4 were also electrodeposited.
Gold chloride electrolytes were used in the early days of Au plating, but today are employed almost exclusively in the electrochemical refining of Au. An extensive investigation of the cathodic behaviour of Au in chloride solutions has shown that the quality of the cathode deposit is strongly influenced by the relative amounts of Au(I) and Au(III) in the solution. The reduction of Au(III) chloride to the metal can be expected to involve the formation of Au(I) as an intermediate species. Under plating conditions, Au will be deposited from both the Au(III) and Au(I) species. Since Au(I) has a more positive plating potential (1.154 V) than Au(III) (1.002 V), a limiting current density for Au(I) will be reached first and it can be expected that the deposits will be of relatively poor quality, i.e., they tend to be bulky and porous. Gold fines will be present in the solution as a result of the following disproportionation reaction:
3AuCl2xe2x88x92=2Au+AuCl4xe2x88x92+2Clxe2x88x92
Detailed studies of the anodic and cathodic reactions have shown that the use of low temperatures and periodic interruption of the current are major factors that can contribute to reduced Au(I) concentration.
Japanese Patent JP 56 136994 to Masayoshi Mashiko describes a process carried out under alkaline conditions and employing a bath composition containing gold, tin and copper and sodium sulphite or potassium sulphite was used as a stabilizer for the gold.
Japanese Patent to S. Matsumoto and Y. Inomata, JP 61 15,992 [86 15.992], (Jan. 24, 1986) discloses a Auxe2x80x94Sn plating bath (pH=3-7) containing KauCl4, SnCl2, triammonium citrate, L-ascorbic acid, NiCl2 and peptone. A 7 xcexcm Auxe2x80x94Sn alloy (20xc2x12 wt % Sn) layer was plated out on a 50 mm diameter Si wafer at 208xc2x0 C. and a current density of 0.6 A/dm2 in 30 minutes using a Pt coated non-consumable Ti anode. The stability of the bath seemed to be the weak link in this process as stability decreased dramatically when the Sn salt was added.
U.S. Pat. No. 6,245,208 (Ivey et al), issued on Jun. 12, 2001 describes a relatively stable, weakly acidic, non-cyanide electroplating solution for codeposition of Auxe2x80x94Sn alloys over a range of compositions, including the technologically important eutectic and near eutectic compositions. In the preferred embodiment, the solution consists of Au and Sn chloride salts, as well as ammonium citrate as a buffering agent and sodium sulphite and L-ascorbic acid as stabilizers.
Ivey et al discusses the use of both direct current and pulsed current power sources and describes relationships between Sn content and average current density, Sn content and pulsed current xe2x80x9cON timexe2x80x9d, and Sn content and pulsed current xe2x80x9cOFF timexe2x80x9d. These relationships indicate that within certain ranges, the Sn content of the resulting Auxe2x80x94Sn alloy will increase with an increase in average current density, pulsed current ON time, and pulsed current OFF time.
Ivey et al also discusses the effect of current density, pulsed current xe2x80x9cON timexe2x80x9d and pulsed current xe2x80x9cOFF timexe2x80x9d upon the quality of the alloy deposit and provides some guidance for optimizing the electroplating process to obtain an alloy deposit of desired composition and quality.
Ivey et al contemplates the application of direct current or pulsed current at a single value of electroplating current density to produce an alloy deposit having a desired Sn content. Unfortunately, however, the relationships amongst the variables, although predictive, are subject to significant scatter due to numerous influences, such as edge effects, local current effects etc. As a result, the exact Sn content of the Auxe2x80x94Sn alloy deposit in Ivey et al is in practice somewhat difficult to control.
As a result, there remains in the art of alloy electrodeposition a need for an electrodeposition process which is capable of providing relatively precise control over the composition or other properties of the alloy deposit.
Preferably this process should be applicable to the electrodeposition of many different alloy systems, including but not limited the gold-tin alloy system.
The present invention is based upon the broad principle that by varying an electroplating current, it is possible to electrodeposit alloy species with distinguishable properties in a controlled manner.
In one aspect the invention is therefore directed at an electrodeposition process for separately depositing layers of at least two alloy species of an alloy to produce a layered composite material. The invention is also directed at a layered composite material comprising a layer of a first alloy species and a layer of a second alloy species, wherein the first alloy species and the second alloy species have distinguishable properties.
The distinguishable properties of the alloy species are due to different alloy phases or combinations of alloy phases being deposited in the alloy species. The invention is therefore applicable to any alloy system in which the alloy is capable of electrodeposition in two or more alloy phases and in which the identity of the electrodeposited alloy phase or phases is dependent upon the electroplating current.
In this specification, the terms xe2x80x9calloyxe2x80x9d and xe2x80x9calloy systemxe2x80x9d indicate substances containing two or more essential elements which are defined by their essential elements and the term xe2x80x9calloy phasexe2x80x9d describes a particular form or phase of a substance which contains the essential elements of the alloy or alloy system. For example, the gold-tin alloy or alloy system contains gold and tin as essential elements and may be produced in several different alloy phases, including for example Au5Sn or AuSn.
In this specification, the term xe2x80x9calloy speciesxe2x80x9d indicates a substance which is electrodeposited by the process using a specific electroplating current, which substance may be comprised of one alloy phase or a combination of alloy phases.
More particularly, the invention may be applied to any alloy system in which two or more alloy phases of the alloy can be selectively electrodeposited by controlling the electroplating current so that an alloy can be electrodeposited as a layered composite material of two or more alloy species which together contain two or more alloy phases. The properties of each particular alloy species are controlled by controlling the electroplating current. The layered composite material is therefore comprised of two or more alloy species and the overall properties of the layered composite material are dependent upon the properties and relative proportions of the different alloy species.
A single alloy species will include those alloy phases of the alloy which are electrodeposited at a selected electroplating current so that a single alloy species may be comprised of one or more alloy phases. Preferably, however, a selected electroplating current electrodeposits primarily or essentially a single alloy phase so that any particular alloy species consists primarily or essentially of a single alloy phase.
Regardless of whether a selected electroplating current deposits one alloy phase or more than one alloy phase, a selected electroplating current should preferably result in the electrodeposition of an alloy species which has consistent properties which are distinguishable from the properties of alloy species which are electrodeposited at a different selected electroplating current. This will facilitate the combination of layers of different alloy species to produce a layered composite material having desired properties.
There is no upper limit to the total number of layers which may make up the layered composite material and the layered composite material may be comprised of as few as two layers.
Regardless of the total number of layers which make up the layered composite material, there should preferably be one or more layers of at least two different alloy species, which alloy species have different properties. The layered composite material is preferably comprised of a plurality of layers of each alloy species.
The layered composite material may be comprised of as few as two alloy phases. Although theoretically there is no maximum number of alloy phases which may be deposited in the various layers of different alloy species, the number of alloy phases present in the layered composite material should preferably be minimized.
Similarly, the layered composite material may be comprised of as few as two alloy species, and although theoretically there is no maximum number of alloy species which may be deposited in the various layers, the number of alloy species present in the layered composite material should preferably be minimized.
The layered composite material is therefore most preferably comprised of two different alloy species, a plurality of layers of each alloy species, and with each alloy species consisting primarily or essentially of a single alloy phase.
The invention may also be applied to the production of an alloy deposit which comprises a single layer of a single alloy species instead of a layered composite material comprised of a plurality of layers of different alloy species. This single alloy species may be comprised of as few as two alloy phases, and although theoretically there is no maximum number of alloy phases which make up the single alloy species, the number of alloy phases comprising the single alloy species should preferable be minimized. Where the invention is applied to the production of a single layer alloy deposit instead of a layered composite material, the single alloy species is most preferably comprised of two different alloy phases.
In a preferred process aspect of the invention, the invention is an electrodeposition process for producing a layered composite material comprised of layers of an alloy, the process using an electroplating circuit comprising a power supply, an electroplating solution comprising ions of the elements comprising the alloy, and an electrodeposition substrate, the process comprising the following steps:
(a) first energizing the electroplating circuit with the power supply to provide a first electroplating current in the electroplating circuit during a first current plating time interval to deposit a layer of a first alloy species of the alloy on the substrate, the first alloy species having first alloy species properties; and
(b) second energizing the electroplating circuit with the power supply to provide a second electroplating current in the electroplating circuit during a second current plating time interval to deposit a layer of a second alloy species of the alloy on the substrate, the second alloy species having second alloy species properties;
wherein the first alloy species properties are distinguishable from the second alloy species properties.
In a preferred product aspect of the invention, the invention is a layered composite material comprising a layer of a first alloy species of an alloy, the first alloy species having first alloy species properties, and further comprising a layer of a second alloy species of the alloy, the second alloy species having second alloy species properties, wherein the first alloy species properties are distinguishable from the second alloy species properties.
The alloy species properties are distinguishable with respect to one or more properties so that by controlling the deposition of each alloy species, the properties of the layered composite material can be controlled by taking advantage of the different properties of the alloy species. The different property or properties of the alloy species may relate to any chemical or physical property. For example, the distinguishing property may be the chemical composition of the alloy species.
Preferably the first alloy species consists essentially of a first alloy phase and preferably the second alloy species consists essentially of a second alloy phase.
The first alloy phase and the second alloy phase will therefore be distinguishable with respect to one or more chemical or physical properties. Preferably the first alloy phase has a first alloy phase composition, the second alloy phase has a second alloy phase composition, and the first alloy phase composition is different from the second alloy phase composition.
The first alloy species and the second alloy species are combined in the layered composite material so that the layered composite material has composite material properties, including a composite material composition. The composite material properties include any chemical or physical properties. The composite material properties will depend upon the first alloy species properties, the second alloy species properties and the relative proportions of the first alloy species and the second alloy species comprising the layered composite material.
The first electroplating current and the second electroplating current may each either be a direct current or a pulsed current. Preferably the first electroplating current and the second electroplating current are both a direct current or both a pulsed current.
The first electroplating current and the second electroplating current are selected having regard to the particular alloy system and the particular electroplating process. The selection of the characteristics of the electroplating currents is guided by an understanding of the relationships between the properties of deposited alloys and electroplating current. Procedures for determining these relationships are taught in U.S. Pat. No. 6,245,208 (Ivey et al) with respect to the gold-tin alloy system. These relationships can be established easily for other alloy systems using the same general procedures.
The first electroplating current is preferably selected so that the first alloy species consists essentially of a first alloy phase and the second electroplating current is preferably selected so that the second alloy species consists essentially of a second alloy phase.
The relative proportions in the layered composite material of the first alloy species and the second alloy species will be dependent upon the first current plating time interval and the second plating time interval. As a result, the first current plating time interval and the second current plating time interval may be selected so that the layered composite material has a desired composite material composition which is obtained by combining the first alloy species and the second alloy species.
The alloy produced by the invention may be any alloy system which may be electrodeposited in different alloy species, which alloy species are dependent upon the electroplating current.
A preferred alloy system for use in the invention is the gold-tin alloy system. Within the gold-tin alloy system, the preferred alloy phases for use in the invention are Au5Sn and AuSn.
The reason Au5Sn and AuSn are preferred alloy phases is because a particularly desirable alloy composition for the optoelectronic and microelectronic industries is the eutectic gold-tin alloy composition, which comprises about 30 at % tin. Au5Sn comprises about 15 at % tin and AuSn comprises 50 at % tin. As a result, it can be readily seen that a combination of Au5Sn and AuSn can readily produce a layered composite material which has a composite material composition comprising anywhere between 15 at % tin and 50 at % tin, thus including the eutectic composition as well as near-eutectic compositions.
For example, by selection of the first current plating time interval and the second current plating time interval, Au5Sn and AuSn can be electrodeposited as a layered composite material to provide a composite material composition of anywhere between about 15 at % tin and 50 at % tin, including between about 25 at % tin and about 40 at % tin, between about 27 at % tin and about 35 at % tin, as well as the eutectic composition.
Where the alloy system is the gold-tin alloy system, the first alloy species therefore consists primarily or essentially of a first alloy phase Au5Sn and the second alloy species consists primarily or essentially of a second alloy phase AuSn.
Electroplating current density is a measure of electroplating current per unit area of electrodeposition substrate. In direct current applications, average current density and peak current density are the same. In pulsed current applications, average current density is a function of peak current density and duty cycle, and duty cycle is a function of electroplating current ON time and pulse cycle period.
It has been discovered that the relationship between average current density and alloy phase in the gold-tin alloy system is such that an average current density of less than or equal to about 1 mA/cm2 will result in the electrodeposition of an alloy species which consists essentially of Au5Sn, while an average current density of greater than or equal to about 2 mA/cm2 will result in the electrodeposition of an alloy species which consists essentially of AuSn. It has also been discovered that an average current density within a range of between about 1 mA/cm2 and 2 mA/cm2 will result in a mixture of Au5Sn and AuSn which varies greatly within that range.
Preferably the first electroplating current and the second electroplating current which are used with the gold-tin alloy system are both pulsed currents. Where the electroplating currents are pulsed currents, the pulsed current ON time, pulsed current OFF time and peak current density are selected first, to provide a suitable average current density to facilitate the electrodeposition of the desired alloy species and alloy phases and second, to provide an alloy deposit which has a suitable quality in terms of grain size and structure.
Fine grained and smooth alloy deposits are generally preferred over coarse grained and rough alloy deposits. The following general trends in alloy electrodeposition are noted:
1. grain structures tend to become less coarse as either average current density or peak current density increase, for current density values below a limiting current density value;
2. grain structures tend to become more coarse as either average current density or peak current density exceed a limiting current density value;
3. grain structures tend to become more coarse with increasing pulsed current ON times; and
4. grain structures tend to become less coarse with increasing pulsed current OFF times.
The limiting current density values for any particular alloy system can easily be determined. In the case of the gold-tin alloy system, it has been found that preferred ranges for the characteristics of the first electroplating current and the second electroplating current are as follows:
The electroplating solution may be any electrolytic solution which includes a suitable solvent containing ions of the elements comprising the alloy or alloy system and which has been suitably stabilized for use as an electroplating solution so that it is capable of codepositing the elements of the alloy or alloy system as two or more alloy species.
As previously indicated, one of the preferred alloy systems for use with the invention is the gold-tin alloy system. In the gold-tin alloy system, a preferred electroplating solution comprises ammonium citrate, a salt of gold soluble in the ammonium citrate, a salt of tin soluble in the ammonium citrate, a gold stabilizer and a tin stabilizer.
Preferably the gold salt is a gold chloride and the tin salt is a tin chloride. More preferably the gold salt is potassium gold chloride (KAuCl4) and the tin salt is tin chloride (SnCl2).
Preferably the gold salt is present in the electroplating solution in the amount of between about 5 g/L and about 15 g/L and the tin salt is present in the amount of between about 5 g/L and about 15 g/L.
Preferably the ratio of gold to tin in the electroplating solution is in the range of about 0.5 to about 3.0 (by weight).
Preferably the gold and the tin are present in a ratio to form the alloy phases Au5Sn and AuSn and are present in a ratio conducive to producing a layered composite material which may contain anywhere between about 15 at % Sn and about 50 at % Sn.
The gold stabilizer and the tin stabilizer may be any substances which will improve the stability of the electroplating solution and facilitate electrodeposition of the layered composite material. Exemplary gold stabilizers include sodium sulfides such as Na2SO3 (sodium sulphite) and Na2S2O3, with Na2SO3 (sodium sulphite) being most preferred, particularly where the gold salt is KAuCl4. A preferred tin stabilizer is ascorbic acid, and in particular L-ascorbic acid.
The preferred electroplating solution may, for example, be prepared in accordance with the method described in U.S. Pat. No. 6,245,208 (Ivey et al) by dissolving a suitable tin salt in ammonium citrate to form a tin solution, dissolving a suitable gold salt in ammonium citrate to form a gold solution, and then combining and mixing the tin solution and the gold solution.
Preferably the gold stabilizer is added to the gold solution and the tin stabilizer is added to the tin solution before the gold and tin solutions are combined.
The layers of the layered composite material may be any thickness, as determined by the lengths of the plating time intervals. Preferably the thickness of the layers is kept relatively small so that the alloy species and alloy phases in the various layers will approximate a homogeneous or completely interspersed structure. Most preferably the thickness of the layers ranges from submicron dimensions ( less than 10 nm) to several microns.